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Pumps

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Pumps Goals Describe how centrifugal and positive-displacement pumps operate and common applications. Calculate system head requirements. Determine head, pump ... – PowerPoint PPT presentation

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Title: Pumps


1
Pumps
2
Goals
  • Describe how centrifugal and positive-displacement
    pumps operate and common applications.
  • Calculate system head requirements.
  • Determine head, pump efficiency, and pump.
    horsepower from a typical centrifugal pump curve.
  • Define net positive suction head (NPSH) and
    understand how it relates to cavitation.
  • Compute NPSH required by a pump.
  • Determine an appropriate pump (impeller diameter,
    efficiency, etc.) for a given required head.
  • Describe how to modify system to operate on the
    appropriate pump curve.

3
Background
Fluid Moving Equipment
Fluids are moved through flow systems using
pumps, fans, blowers, and compressors. Such
devices increase the mechanical energy of the
fluid. The additional energy can be used to
increase
  • Velocity (flow rate)
  • Pressure
  • Elevation

4
Background
Pump, fan, blower, and compressor are terms that
do not have precise meaning. Generally pumps
move liquids while fans, blowers and compressors
add energy to gasses. Pumps and fans do not
appreciably affect the density of the fluids that
they move and thus incompressible flow theory is
applicable.
5
Centrifugal Pumps
Most common type of pumping machinery. There are
many types, sizes, and designs from various
manufacturers who also publish operating
characteristics of each pump in the form of
performance (pump) curves. The device pictured
on the cover page is a centrifugal pump. Pump
curves describe head delivered, pump efficiency,
and net positive suction head (NPSH) for a
properly operating specific model
pump. Centrifugal pumps are generally used where
high flow rates and moderate head increases are
required.
6
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7
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8
Impeller
9
Positive Displacement Pumps
To move fluids positive displacement pumps admit
a fixed volume of liquid from the inlet into a
chamber and eject it into the discharge.
Positive displacement pumps are used when higher
head increases are required. Generally they do
not increase velocity.
10
Pump Specification
Recall Mechanical Energy Balance
Both equations describe work that must be
supplied to system
11
Pump Head
What happens if the MEB is multiplied through by
g (gc/g)?
What are the units (SI)?

W/g has units of length and is known as the pump
head
12
Example
Why do we choose point 2 rather than 3 for MEB?
What kind of valve to uses to control flow rate?
13
Example
Mechanical Energy Balance (in terms of head)
14
Head vs. Flow Rate
Quadratic In V or q
15
System Response
  • What happens when flow control valve is closed?
  • Resistance (f) increases
  • Flow rate decreases
  • Need more head to recover flow rate

16
System Response
Constant Flow Response
Valve Open
Valve Closed
Constant Head Response
17
Pump Curves
Pump manufacturers supply performance curves for
each of their pumps. These are normally referred
to as pump curves. These curve are generally
developed using water as the reference fluid. The
following can be read directly from a pump curve
  • Head vs. flow rate information for any fluid
  • Pump efficiency for any fluid
  • Pump horsepower for system operating with water

18
Pump Performance Curves
Efficiency
NPSH
Impeller Diameter
Developed Head
Horsepower
Flow Rate
http//capsicum.me.utexas.edu/ChE354/resources.htm
l
19
Power Input
  • For fluids other than water

20
Power Input
Easier Way
Note A less dense fluid requires less horsepower
21
Example
Q 300 gpm
Di 10
Head(ft) ?() P(hp)
Di 10
Head(ft) ?() P(hp)
Di 10
Head(ft) ?() P(hp)
22
Goulds Pump Curves
Manufacturers provide series of pumps to cover
broad ranges of capacities, heads, and suction
and discharge piping diameters. Most pumps can
be equipped with different diameter impellers and
can be operated at different speeds to change
capacities.
The curves provided are for a few variations of
the Goulds model 3196 process pump. Each curve
corresponds to a specific pump and a specific
RPM. Pump sizes are denoted with 3 numbers.
3 x 4 - 7
Suction Diameter Inches
Discharge Diameter Inches
Casing Diameter Inches
Note Try to match process piping diameters
with the pump discharge and suction diameters.
23
Pump Selection
Goal is to find a pump whose curve matches the
piping system head vs. flow rate curve. We can
superimpose the previous head-flow rate curve on
the manufacturers pump curves. To select a
specific pump from a product line, find the pump
with the highest efficiency that does not require
the use of the largest impeller diameter. This
will allow for future production expansions.
Suppose that we have a process that requires a
flow rate of 300 gpm and has a head requirement
of 60 ft. at that flow rate. Can a 3x4-10 model
3196 Goulds pumps be used?
24
Example
Impeller Diameter
For Desired Q Head
How do can you force the system to operate on the
pump curve?
25
Net Positive Suction Head (NPSH)
Associated with each H-Q location on the pump
curve is a quantity that can be read called NPSH.
An energy balance on the suction side of the
fluid system (point 1 to pump inlet) with pinlet
set to the vapor pressure of the fluid being
pumped gives a quantity called NPSHA (net
positive suction head available).
26
Net Positive Suction Head
The requirement is that
Otherwise (if NPSHA lt NPSHpump), the pressure at
the pump inlet will drop to that of the vapor
pressure of the fluid being moved and the fluid
will boil. The resulting gas bubbles will
collapse inside the pump as the pressure rises
again. These implosions occur at the impeller
and can lead to pump damage and decreased
efficiency.
Cavitation
27
NPSH
Do not use NPSH to size or select a pump unless
all else fails. Pump selection is governed by H
vs. Q requirements of system. When NPSHA is too
small, it might be increased by
  • Increasing source pressure (not usually feasible)
  • Cooling liquid to reduce vapor pressure (not
    usually feasible)
  • Raise elevation of source reservoir
  • Lower elevation of pump inlet
  • Raise level of fluid in reservoir

28
If NPSHA Cant Be Increased
If the pump must be modified to achieve proper
NPSH
  • Larger slower-speed pump
  • Double suction impeller
  • Larger impeller eye
  • Oversized pump with an inducer

29
Example
30
Pump Selection from Many Choices of
Characteristic Curves
  1. Examine pump curves to see which pumps operate
    near peak efficiency at desired flow rate. This
    suggests some possible pipe diameters.
  2. Compute system head requirement for a few
    diameters.
  3. Compute V for some diameters. For water V in the
    range of 1 10 ft/s is reasonable (see ahead).
  4. Re-examine pump curves with computed head and
    pipe diameters. This may give a couple of
    choices.
  5. Pick pump with highest efficiency.

31
Selection of Pipe Size
Optimum pipe size depends mainly on the cost of
the pipe and fittings and the cost of energy
needed for pumping the fluids. Cost of materials
increase at a rate proportional to about D1.5,
while power costs for turbulent flow varies as
D4.8. One can find correlations giving optimum
pipe diameter as a function of flow rate and
fluid density, however the optimum velocity is a
better indicator as it is nearly independent of
flow rate.
32
Optimum Pipe Size
For turbulent flow of liquids in steel pipes
larger than 1 in.
33
Remember
  • Maximize pump efficiency
  • Power input (hp) should be minimized if possible
  • Selected impeller diameter should not be largest
    or smallest for given pump. If your needs change
    switching impellers is an economical solution
  • NPSH required by the pump must be less than NPSHA

34
Variable Speed Pumps
Advantage
Lower operating cost
Disadvantage
Higher capital cost
RPM2
35
Affinity Laws
In some instances complete sets of pump curves
are not available. In this instance the pump
affinity laws allow the performance of a new pump
to be determined from that of a similar model.
This can be useful when modifying the operating
parameters of an existing pump.
36
Affinity Laws
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